Another meteorite with a pedigree found on the basis of data taken by the European Fireball Network

Figure 1. Part of the all-sky image showing the fireball of July 10, 2018 obtained by the automatic digital all-sky fireball camera at the Czech station Přimda. The bolide is very low above the western horizon. (photo: Astronomical Institute of the CAS)

Press release 2. 10. 2018

On Tuesday night, July 10, 2018 shortly before half past eleven local time (CEST), a large area around the middle Rhine, around the borders of Germany and France, especially the regions of Baden-Württemberg, Rhineland-Palatinate, Alsace and Lorraine, was illuminated by a very bright meteor - bolide. With the exception of much of France and the smaller areas of Germany (mainly in the south-west part), observations of this bolide were complicated by relatively extensive and compact clouds, which covered a significant part of Western and Central Europe. Therefore, this bolide, which for a short time was even brighter than the full Moon, did not attract such large attention as it is usually the case for such bright events. This was also because its luminous atmospheric trajectory was very steep and therefore relatively short, both in length and in the duration of the whole phenomenon. Fortunately, not only in the vicinity of the bolide, but also in our territory, especially in Western Bohemia, there were partly clear skies at some places. It soon proved to be the decisive factor for correct explanation and description of this very rare natural event. This bolide was recorded by instruments of the Czech and German parts of the European Fireball Network, from sites where it was at least partly clear at the time of its passage. Thanks to these records, this bolide could be reliably and accurately described. Among others, we were able to predict that it had ended with meteorite fall and determine the area where they fell and how large they should be. All analyses of the available records were carried out by a team of scientists of the Interplanetary Matter Department of the Astronomical Institute CAS in Ondřejov in a very short time. The great success of this international scientific project is that shortly after sending the information about the impact area to the German colleagues, the meteorites were recovered by a German searching team exactly in the predicted locations for given meteorite mass. This event therefore fall into the rare category of meteorites “with pedigree”, i.e. meteorites with known trajectory and pre-impact heliocentric orbit.

Figure 2. Light curve of the bolide EN100718 Renchen taken by the radiometer of the DAFO with time resolution of 5000 samples/s from the Czech station Přimda. The light curve is among others very important for quantitative analysis of fragmentation history of the recorded bolide. (graphic: Astronomical Institute of the CAS).

The fireball (hereinafter referred to as EN100718 Renchen) was recorded by the digital automatic all-sky cameras (DAFO) at the Czech stations Přimda (see Figure 1) and Churáňov. The images from these cameras enabled us to pre-determine not only where the bolide flew, but above all to find out that it was the case which had certainly ended by a meteorite fall. These stations are part of the European Fireball Network (EN), which covers the territory of Central Europe and whose center is in the Astronomical Institute of the Czech Academy of Sciences in Ondřejov. The view of the bolide very low, only 10.4 °-0.8 ° (!) above the western horizon taken by the all-sky digital camera at the station of the Czech Hydrometeorological Institute at Přimda is in Figure 1. The detailed light curve of the bolide with a very high time resolution (5000 samples/s) is other essential information provided by our instruments. These precise photometers are part of each DAFO and, in addition to the bolide light curves, they also record very precisely the time of the recorded phenomenon. From all these records we determined within one day where the phenomenon occurred, how powerful it was, and that it certainly ended by a meteorite fall. We were able to obtain this basic information despite the fact that our cameras were very far from the bolide (365 to 420 km). However, it was clear from the beginning that it would be very helpful to get some additional records of this bolide from a closer distance that would refine our results and also complement them. A few hours after the bolide, we contacted the administrator of the German part of the EN, Dieter Heinlein, and informed him about the significance of this bolide. We asked him for data from the cameras, which in Germany belong to the DLR (German space agency). Like in the Czech part of the EN, these cameras take all-sky records, but in a significantly lower resolution and in the vast majority still on the classic film. As it later turned out, only one camera in Germany and one in France (digital version of the same camera) recorded this bolide in a way usable for further analysis. For a full description of the bolide, the image from the station 78 Osenbach (Alsace, France) proved to be a very important. Although the last third of the bolide including also its brightest part was completely behind the clouds, which means that information about end of the bolide was completely missing, this picture was still very important for the analysis because it contains time marks on the fireball trail. These regular interruptions (breaks) are caused by a rotating shutter and they serve for determination of the speed of the fireball.

Figure 3. Image of the bolide EN100718 Renchen taken by a mirror all-sky camera of the German part of the European Fireball Network at the station 87 Gernsbach (photo: DLR, Institute of Planetary Research, Germany).

Another advantage of this record consists in its fast availability because we obtained it from our German colleagues already the second day after the fireball. It enabled us, together with the Czech records, to determine more reliably all important parameters of the meteoroid atmospheric trajectory, elements of its heliocentric orbit and also model the impact area of meteorites. Additionally, after the film was developed and sent to the Ondřejov, a picture from the station 87 Gernsbach, which was the closest to the bolide trajectory (only 30 km away – ground distance) was used. However, similarly to the pictures from Czech stations, time marks were not measurable on this this image which means that it was not helpful in speed determination. Moreover, as there were almost no reference stars, it went very hard to reduce. On the other hand, it is a picture where bolide is best seen and finally served for the determination of the terminal height. As seen in Figure 3, due to bad weather, almost nothing except the bolide is visible on this image. In addition to these records, we still received pictures from webcams from Karlsruhe and Oberwolfach in Germany and from Schafmatt in Switzerland. Unfortunately, the further analysis showed that these still images could not be used for independent determination of the bolide trajectory so they were not used for the final calculation. The whole phenomenon was thus described exclusively from the camera records of the European Fireball Network.

What exactly happened on Tuesday, July 10, 2018 over Western Germany?

Exactly at 21:29:49 universal time (23:29:49 local time), not too big meteoroid about the size of a football ball entered the Earth's atmosphere and started to weakly radiate at a height of 80 km. Thanks to the very steep trajectory with the slope about 80 degrees to the surface, its brightness increased very quickly and reached maximum magnitude of –13.4 (recomputed for a distance of 100 km), i.e. more than the brightness of the full Moon. The projection of the atmospheric trajectory is shown in Figure 4 and is relatively short (only 13 km) because of the steep trajectory. In fact, the luminous path of the bolide was 63 km long and the body travelled it in 4 seconds. The whole event took place almost exactly above the German town of Renchen in Baden-Württemberg. The meteoroid entered the atmosphere with a speed of slightly less than 20 km/s and gradually decelerated and also fragmented. Thanks to the relatively low speed, steep trajectory, reasonably large initial mass and sufficient strength of the material, this bolide has ceased to shine only 18 km above the ground, just slightly west of the town of Renchen near the Rhine River, which forms the border between Germany and France. Such a deep penetration is rare, which is demonstrated by the fact that such a low terminal height was observed only exceptionally during several decades of our systematic observations. Moreover, this deep penetration clearly means that meteorites have been dropped. This fact, in combination with one distinctive flare that we observed on the bolide’s light curve at a height of 28 km (see Figure 2) and which represents a significant fragmentation event, it is evident that a larger number of fragments of wide range of masses from grams to kilograms reached the surface. The impact area of meteorites of all sizes lies exclusively on the territory of Germany, mainly in the area of the town of Renchen (see Figure 5).

Figure 4. Projection of the luminous atmospheric trajectory of the bolide EN100718 Renchen to the Earth’s surface (yellow arrow). The real length of the photographed trajectory was 63 km and the meteoroid travelled it in 4 seconds. (graphic: Google/ Astronomical Institute of the CAS).

Smaller fragments are in the eastern part of the predicted area and their weights rises westwards. The biggest meteorites should be in a narrow strip west of Renchen. The shift of the meteorite impact area relative to the bolide trajectory is caused by the air flow (jet stream) in the higher atmospheric layers. The dark phase of the flight, when the fragments of the original body moved roughly at the speed of free fall, lasted about 2 minutes for the end point of the trajectory (altitude 18 km) and a kilogram meteorite and even 5 – 10 minutes for small 1 – 10 gram meteorites originated from the main flare (altitude 28 km). Therefore, stratospheric winds have significant influence on the final position of the impact area for individual meteorites even for such a favorably steep trajectory, which significantly reduces the impact area, as was the case with the Renchen fall.

Before its collision with the Earth, the meteoroid orbited the Sun on a low eccentric orbit, which was moderately inclined to the plane of the ecliptic (i.e. Earth’s orbit). In perihelion, the meteoroid was only a little closer to the Sun than the orbit of our planet Earth, and in aphelion it reached the inner parts of the main belt of asteroids. It was therefore a small part of an Apollo type asteroid originating from the inner part of the main belt of asteroids.

Figure 5. Schematic representation of the impact area. Small fragments are expected in the larger eastern part and fragment mass will increase toward west and northwest. The largest fragments are predicted to be found in the narrow strip in the northwest part. Locations of the recovered meteorites are marked by points M1 (11.9 g), M2 (955 g), M3 (20.6 g), and M4 (4.8 g). (graphic: Google/Astronomical Institute of the CAS).

Meteorite findings according to predictions

Figure 6. The first recovered meteorite Renchen (M1) with mass 11.9 g in finding position and in detail. It was found two weeks after the fall on July 24, 2018 by Ralph Sporn and Martin Neuhofer. (photo: Ralph Sporn and Martin Neuhofer, Pavel Spurný).

Shortly after confirming that it was a significant fall of meteorites, the first attempts to find them began. This effort is mainly coordinated on the German side by Dieter Heinlein, who has been working with us on a project of the European Fireball Network for a long time. The first description of the impact area, together with the most appropriate search strategy (due to the anticipated numbers of the various mass groups of meteorites, we first focused on the area of small meteorites originated from the main fragmentation) was provided to colleagues in Germany on Saturday, July 14, 2018. The first search, attended by two staff members of the German part of the European Fireball Network, the very experienced and reliable searchers Ralph Sporn and Martin Neuhofer, who successfully participated in finding of several previous cases, took place from Sunday, July 15 until Tuesday, July 17. These first attempts mainly served to explore the whole area and to identify suitable places for subsequent systematic searches. On the basis of this experience, both searchers continued to systematically search in suitable areas from the following Sunday, July 22. Despite very difficult conditions (mainly high vegetation, high temperatures and inaccessibility of some areas) already on Tuesday, July 24 around noon their efforts were crowned with success, when they found a first meteorite weighing 11.9 grams (see Figure 6). This small meteorite was largely covered with fresh black fusion crust and evidently belongs to the bolide of the July 10. The finding position and the mass of the meteorite correspond very well to the predictions, which were still gradually improved by a more detailed model of the wind profile, which we obtained later after the fall. This also made possible to refine the position of the impact area either for one main piece or in the case of its breakup near the end of the trajectory for several smaller pieces originated from this breakup. Despite considerable difficulties with reduction also the image from the station 87 Gernsbach contributed to better description of the bolide especially near the end of its trajectory. From the detailed modelling of fragmentation and size of individual fragments, we got a better picture of the possible distribution of larger meteorites. We sent this area of larger pieces to both searchers in the field on Monday July, 30 around noon. They started their third search campaign again on Sunday and they were scheduled to continue until Wednesday August 1. As it turned out that the area for the main piece lies in a forest with dense and high undergrowth, for Tuesday's search, they chose a strategy to search for those places that are well searchable and lying on the most probable line, which would correspond to the scenario that the main piece fragmented into a few about one kilogram sized pieces. This soon proved to be a very good and lucky choice. In a well searchable meadow, these two searchers found a meteorite weighing 955 grams, which was only minimally sunk into the soil and which corresponded very precisely to the predicted position for the given weight – see Figure 7. The vast majority of its surface was again covered with black fusion crust, which bore traces of impact to the loamy soil. A small part was missing (maximum of about 100 grams) and it had to be separated during the dark flight, as there were no apparent thermal influence on the broken surface. This hypothesis was confirmed by another finding almost exactly a month later, when another group of searchers, who received information about the impact area from D. Heinlein later, found several small broken fragments. These meteorite pieces, weighing together 20.6 grams, were found by Lukasz Smula, Magda Skirzewska and Thomas Kurtz on Saturday, September 1 on a forest path, in a position not reachable by small fragments from a higher fragmentation point (e.g. the main explosion).

Figure 7. The largest recovered meteorite Renchen (M2) with mass of 955 g found 3 weeks after the fall by Ralph Sporn and Martin Neuhofer on July 31, 2018. The picture was taken shortly after the find and except the meteorite also small pit created by its impact is visible. (photo: Ralph Sporn and Martin Neuhofer).

During their eighth field trip to the Renchen area, after 26 tiring and exhausting searching days, Ralph Sporn and Martin Neuhofer achieved another fantastic meteorite find on Sunday, September 30. They have found a meteorite individual, weighing 4.8 grams, hanging in the bird's net which was covering an apple fruit plantation east of Renchen, exactly in the predicted area for a given mass. This meteorite piece didn't reach the Earth's surface actually, but was recovered from a height of 2.5 meters above ground (see Figure 8).

Figure 8. On September 30, 2018 Ralph Sporn and Martin Neuhofer recovered a meteorite piece (M4) weighing 4.8 g east of Renchen, which was hanging in the bird's net covering an apple fruit plantation. Detailed image of the meteorite is inserted. (photo: Ralph Sporn and Martin Neuhofer, Dieter Heinlein).

The meteorite classification was carried out by Dr. Addi Bischoff and his PhD student Markus Patzek of the Institute for Planetology at the University of Münster, Germany. The meteorite proved to be an ordinary chondrite of type L5-6. It is a breccia, i.e. meteorite composed of different rock types. Also the composition clearly confirmed the asteroidal origin of the initial meteoroid. The unofficial name of the meteorite is Renchen.

The discovery of meteorites perfectly confirmed the whole scenario of this fall, which we have inferred from the analysis of all available records suitable for analysis taken by instruments in the Czech and German parts of the European Fireball Network. It is one of the greatest achievements of this long-term international project, which was founded by Dr. Zdeněk Ceplecha in 1963. The European Fireball Network was the first such experiment in the world and is the only one remaining in full operation for so long.

There are currently known about 30 meteorites with pedigree, i.e. those with instrumentally determined trajectory and orbit in the Solar System. In more than half of the cases, including the very first one, the Příbram meteorite of April 7, 1959, the analysis was performed by the scientists of the Astronomical Institute of the Czech Academy of Sciences. According to their calculations, for example, meteorites Ždár nad Sázavou from 9 December 2014, Hradec Králové from 17 May 2016 or Stubenberg in Germany from 6 March 2016 were also recently found.

Acknowledgements: We thank to Dr. Radmila Brožková from the Czech Hydrometeorological Institute in Prague and Dr. Gerd Baumgarten of the Leibniz-Institute of Atmospheric Physics, Kühlungsborn for providing wind models for this fall. We appreciate that Lukasz Smula has donated some samples for meteorite classification and further analyses.